A resistive temperature device (RTD) in operation responds to a change of temperature in its immediate vicinity (such as the windings of a motor) with a change in its resistance. The relationship between the change of resistance relative to a change in environmental temperature for each type of RTD is known. A current is applied to the RTD and the voltage drop across the RTD in response is determined. The resistance of the RTD is then calculated. The calculated resistance is then typically compared to a look-up table relating resistance to temperature to determine the environmental temperature. The RTD depends upon an accurate current source to provide accurate temperature determinations. RTD modules to which a plurality of RTDs is typically connected perform resistance determinations and transmit data from the RTDs. The RTDs typically are located in transformers, circuit breakers, motors, generators or other similar apparatus which require temperature monitoring.
RTDs are often located in environments which are signal-noisy, such as, in particular, generators or motors which produce AC signal noise in operation, typically 60 Hz. The 60 Hz AC signal present in the generator is induced in the RTD leads in addition to the RTD DC voltage signal present thereon produced in response to the current from the constant current source. If the induced AC signal exceeds the operating range of the RTD module, the measured signal will be incorrect.
A common technique to eliminate, i.e. block, the AC signal noise from the RTD leads is the use of a low pass filter (LPF) to which the voltage signal (with the AC signal) is applied prior to measurement of the RTD voltage. The low pass filter will block selected frequencies and can be designed to block 60 Hz. However, the LPF has a time constant due to its resistance and capacitive elements, with the time constant being inversely proportional to the cutoff frequency. The lower the cutoff frequency, the longer the time constant.
The low pass filter delays the rate at which the individual RTDs can be sampled for the voltage value by the module, because of its time constant. The voltage cannot be sampled until the filter is charged for accurate results. For instance, if the time constant of the low pass filter requires one second to be fully charged, sampling should not occur for the RTD associated with the low pass filter until the one second has passed. As a result, the rate of sampling the entire group of RTDs served by a particular module will be increased significantly.
The total time between successive samplings of a given RTD for a given module, when the module serves a plurality of RTDs, is referred to as the “refresh rate”. With the use of a low pass filter for each RTD, which eliminates the AC noise, the refresh rate will increase significantly. In one typical example, for a module serving four RTDs, the refresh rate may increase from 1 second to 10 seconds. Such an increase in refresh rate is unacceptable in many cases, since it allows too much time for undetected significant increases in temperature.
Hence, while a low pass filter is effective in eliminating AC noise, the resulting substantial increase in refresh time is a significant disadvantage. Hence, it is desirable to significantly reduce the refresh time when low pass filters are used with RTDs.